The present invention relates to a pelletizing apparatus for thermoplastic polymers. More particularly, it relates to an improvement in a vertical underwater type pelletizer-cutter apparatus utilizing a hydraulic suction force for the pelletization of thermoplastic polymers.
For pelletization of thermoplastic polymers, there have been known two processes, i.e. the one wherein a thermoplastic polymer is extruded through a nozzle in a piston type or screw type extruder, the resulting strands are cooled at a cooling zone and the cooled and solidified strands are cut by a cutter means to give pellets, and the one wherein a thermoplastic polymer is extruded through a nozzle which has the same cross-section as the desired pellet, the resulting strands are cut in pellets immediately after extrusion and while not completely solidified and the resultant pellets are cooled with air or water to perfect the solidification of the pellets.
In the former process, there is frequently used a vertical underwater type pelletizer-cutter apparatus utilizing a hydraulic suction force, which comprises a nozzle portion, an injector portion, a cooling portion and a cutter portion. Thus, a thermoplastic polymer in a melt state is extruded through a nozzle portion, and the resulting strand of the thermoplastic polymer is introduced into an injector in the injector portion and lead through a guide tube in the cooling portion where the strand is cooled and solidified with water introduced from the injector into the guide tube. The strand is combined together with the cooling water in the guide tube to the cutter portion where it is cut to produce pellets, which are discharged and then dried. In the above steps, the strand of the thermoplastic polymer in a melt state coming through the nozzle often accumulates at the entrance or top of the injector due to the swinging movement of the strand, the variation of the amount of the cooling water introduced into the injector portion, problems produced in the cutting operation and the like and, because the cooling water which is conveyed through the injector solidifies immediately, causing clogging. Once the clogging occurs, the accumulation of the polymer at one injector readily grows to adjoining injectors and ultimately covers all of the injectors so that it becomes necessary to stop the operation in order to romove the accumulated and solidified polymer. This not only necessitates the consumption of enormous labor and time but unavoidably induces deterioration of the quality of polymer being produced.
For preventing the clogging of the polymer at the injector portion, it has been the usual practice to rely on an operator's observation. In consideration of the limit of human observation ability and the savings in labor, it is more desirable to automatically detect the clogging of the injector at the very beginning of the operation, thereby eliminating such problems at an early stage.
As the result of an extensive study, it has now been found that normally the upper portion of the guide tube does not fully fill with water, and thus when the clogging begins to start at the injector, the pressure in the said space is reduced. The pressure reduction reaches its maximum several seconds after the clogging up of the injector and then gradually reaches a certain constant level with the lapse of time. The time in which the maximum reduced pressure is reached is usually from about 3 to 6 seconds, although it may be varied with the amount and temperature of the cooling water, the type of injector, etc.
Based on the above finding, the present invention provides a vertical underwater type pelletizer-cutter apparatus utilizing a hydraulic suction force for the pelletization of thermoplastic polymers comprising a nozzle portion having a plurality of nozzles through which a thermoplastic polymer in a melt state is extruded in strands, an injector portion having a plurality of injectors through which the strands are introduced together with water, a cooling portion having a plurality of guide tubes through which the strands are conveyed together with said water to cool and solidify the polymer and a cutter portion where the cooled and solidified strands are cut into pellets, wherein means are provided for monitering the pressure of the space in the upper portion of the guide tube and for automatically stopping the extrusion of the thermoplastic polymer when the monitering means detects an abnormal reduction in the pressure.
In the apparatus of the present invention, there is no particular restriction on the monitering means. Since, however, the degree of the pressure reduction resulting from the clogging of the injector portion is not very high, it is preferable to use a device which is capable of sensitively detecting even a minute pressure variation. As such device, there may be used one which can measure the pressure due to a difference in liquid levels, e.g. a manometer; one which can measure a pressure due to elasticity deformation of a membrane caused by pressure difference e.g. Bourdon tube vacuum gauge, diaphragm type vacuum gauge, bellows type vacuum gauge, and the like. The connection of the device with the guide tube may be accomplished by the use of a fine tube, preferably at the upper portion of the guide tube.
There is also no particular limitation on the automatic control means for stopping the extrusion of the thermoplastic polymer upon an detection of the abnormal pressure reduction by the monitering means. For stopping the extrusion, a cock valve, or the like is usually employed. In the apparatus of the present invention, the use of an automatic valve utilizing air, electricity, oil or the like (e.g. diaphram type regulating valve, electric valve) is desirable.
The connection of the monitering means with the automatic control means may be achieved by any conventional manner. For instance, an electric circuit, a compressed air duct or their combination may be adopted. Preferably, the monitering means on one injector may be connected with the automatic control means on the extrusion portion or nozzle corresponding to the said injector. However, one monitering means is not always required to be connected with one automatic control means and thus it may be connected with two or more automatic control means.